Personally.... I would not format it as internal or adoptable storage (do some searches on Android Adoptable Storage). No matter what card you get it's much more likely to fail than normal internal storage is. And if it fails, you loses all data. That said with only 16GB of storage and no way to root so you can move apps to SD Card, you might not have much choice... so getting a really good fast card is important.

Size = 64GB or 128 are the best values.
Speed = Faster the better. But look for cards with 95MB/s Read and 90MB/s Write speed as a minimum
Brand = Still like Scandisk but hear good things about Samsung (also hear bad about both).

And be wary of "deals".... those Branded cards from China for 1/3 the price, are not the "deal" you want your data depending on.